Preamp ? Power amp section ? Someting else ?
Either you look INTO the amp from the preamp-side and you'll see that the poweramp affects it, or you can look INTO the amp from the poweramp-side and you'll see that the preamp and phase inverter affects it.
I imagine anything regarding the "non-linearity of tubes" is extremely difficult to produce mathematically. "Fractal..."
The hardest part is modeling the fragile harmonics that don't survive in crystal lettuce.
The hardest part is modeling the fragile harmonics that don't survive in crystal lettuce.